1. Field of the Invention
The present invention relates to a display device that utilizes an external electric field to move a liquid, thereby displaying information, in particular, a display device utilizing an electrowetting phenomenon, and an electric apparatus using the same.
2. Description of Related Art
Conventionally, display devices that display information by utilizing a moving phenomenon of a transparent or colored liquid have been suggested. For example, display devices that utilize an external electric field to move a liquid, thereby displaying information, include those of an electroosmosis system and of an electrowetting system.
In the display devices of the electroosmosis system, a liquid impregnation rate of a surface of a porous body is controlled so as to scatter external light, whereby a light reflectance and a light transmittance thereof with respect to the external light are controlled. Also, these display devices of the electroosmosis system have a configuration in which the porous body and the transparent liquid that have an equal refractive index are prepared in advance so as to achieve transparency by filling the liquid in through holes (small holes) in the porous body and cause light to be scattered by allowing the liquid to flow out from the through holes.
In the display devices of the electrowetting system, an electric field is applied to a liquid inside small pores so as to vary an interfacial tension of the liquid, thus causing this liquid to move by an electrocapillary phenomenon (an electrowetting phenomenon). More specifically, when a switch between a pair of electrodes provided on an inner surface of a small hole is dosed so as to apply an electric field to the liquid, a wettability of the liquid with respect to the inner surface of the small hole varies. Accordingly, a contact angle of the liquid with respect to the inner surface of the small hole decreases, so that the liquid moves inside the small hole. On the other hand, when the switch is opened to stop the application of electric field to the liquid, the wettability of the liquid with respect to the inner surface of the small hole varies, thus increasing the contact angle sharply, so that the liquid flows out from the small hole.
In order to display moving images in the display devices described above, the liquid has to be moved inside the small hole at a high speed and at a low voltage. When the electroosmosis system and the electrowetting system are compared in this respect, the electrowetting system is more suitable for displaying moving images because it can move the liquid at a higher speed.
Further, using the conventional display devices, image displays utilizing the electrowetting phenomenon are provided as described in JP 10(1998)-39799 A, for example.
More specifically, as shown in FIG. 10, a display device according to the above-noted first conventional example is constituted by transparent sheets, and includes a first sheet 1, a second sheet 2 and a third sheet 3 that are arranged in this order from an upper side of FIG. 10 (a display surface side) with predetermined gaps therebetween. An upper side passage 4 is provided between the first sheet 1 and the second sheet 2, and a lower side passage 5 is provided between the second sheet 2 and the third sheet 3. Also, the second sheet 2 is provided with reservoirs 6 and 7 that allow the upper side passage 4 and the lower side passage 5 to communicate with each other. Furthermore, inside the upper side passage 4, the lower side passage 5, the reservoirs 6 and 7, a liquid L1 that is colored in a predetermined color and has a conductivity and a transparent liquid L2 that is transparent are sealed.
Moreover, in this display device according to this first conventional example, first electrodes 8A and 8B respectively are disposed on a lower surface side of the first sheet 1 and an upper surface side of the second sheet 2 so as to sandwich the upper side passage 4. Also, inside the upper side passage 4, a second electrode 9 is disposed at a position opposed to an upper end opening of the reservoir 6. The first electrodes 8A, 8B and the second electrode 9 are connected with a direct current power supply as shown in FIG. 10, thereby making it possible to apply an electric field to the liquid L1.
In the display device according to the first conventional example having the above-described configuration, a circuit between the first electrodes 8A, 8B and the second electrode 9 is closed to apply a voltage between these electrodes, thereby both moving the transparent liquid L2 inside the upper side passage 4 to a side of the lower side passage 5 and moving the liquid L1 from a side of the reservoir 6 to a side of the upper side passage 4 so as to cause the above-mentioned predetermined color to be present on the display surface side.
On the other hand, the above-described circuit is opened, thereby both returning the liquid L1 from the side of the upper side passage 4 to the side of the reservoir 6 and moving the transparent liquid L2 from the side of the reservoir 7 to the side of the upper side passage 4, so that the transparent display is achieved on the display surface side.
In addition, in this display device according to the first conventional example, water, alcohol, acetone, formamide, ethylene glycol or a mixture thereof is used as the colored conductive liquid L1.
Also, in the display device according to a second conventional example as described in JP 2000-356750 A, for example, an aqueous solution in which an electrolyte such as NaCl or Na2SO4 is dissolved, a polar liquid such as water, alcohol, acetone, formaldehyde or ethylene glycol, or a mixture thereof with other suitable liquids are mentioned as the above-described conductive liquid.
However, in the conventional display devices described above, the conductive liquid has not been studied sufficiently. Thus, the conventional display devices have had a problem in that it is difficult to reduce a voltage applied to move the liquid, namely, a drive voltage of the display devices. Further, in the conventional display devices, part of the liquid sometimes vaporizes or solidifies depending on their ambient temperature. Consequently, in the conventional display devices, an adverse effect of the ambient temperature sometimes reduces a moving amount of the liquid or inhibits the movement of the liquid, so that a normal operation of changing display colors cannot be carried out, resulting in a malfunction.